1. Field of the Invention
The present invention relates to a pulling device for warp yarns in crochet galloon looms of the type comprising a main shaft operated in rotation, at least a pair of pulling rollers engaging a manufactured article downstream of knitting means designed to produce said manufactured article by interlacing weft yarns with warp yarns, and at least a first driving mechanism transmitting the rotatory motion of the main shaft to the pulling rollers.
2. Prior Art
It is known that in crochet galloon looms the pulling of the warp yarns is conventionally performed by means of pulling rollers acting downstream of the knitting means on the manufactured article they have produced. In greater detail pulling rollers rotated to an appropriate speed engage the manufactured article so as to move it apart from the knitting means as the manufactured article is being produced.
Due to the fact that the manufactured article is moved away from the knitting means the warp yarns are pulled along towards the knitting means to be interlaced with the weft yarns, being acted upon by said knitting means as the production of the article is going on.
For the purpose of performing a regular knitting it is of the greatest importance that the warp yarns should be submitted to a constant and appropriate tensioning. Said tensioning is presently accomplished by means of specific members individually engaging the warp yarns upstream of the loom and sometimes by the use of driving devices disposed upstream of the knitting means. Each tensioning member is substantially arranged so as to submit the respective warp yarn to a forced passage between two dish-shaped elements resting upon each other. While the yarn is sliding a frictional force is generated between the yarn and the dish-shaped element, which force counteracts the action of the pulling rollers giving the yarn the appropriate tensioning.
The pulling of the warp yarns carried out on the basis of the known art gives rise to several problems.
One of them is due to the fact that the frictional force counteracting the pulling of the yarns and, as a result, the extent of the warp tensioning, can vary if impurities are laid down on the tensioning members or any other member designed to guide the yarns. Even if a minimum variation in tensioning occurs, an irregular knitting is produced, which involves defects in the manufactured product. In order to avoid the above mentioned drawback it is presently necessary to submit the take-up members and all elements designed to guide the yarns to cleaning and servicing operations at predetermined time intervals and obviously said operatons involve long down times.
Systems of the known art also suffer the disadvantage that if the operation of the loom stops, the tensioning of the yarns is subjected to be completely altered once the loom is started again.
In greater detail, when the loom is operated anew, the warp yarns are for a short time subjected to a tensioning higher than that of a normal operation. This is due to the fact that in order to restore the normal sliding of the warp yarns, it is necessary to overcome static frictions existing between the yarns and the elements on which they slide. These static frictions are higher than the dynamic frictions which can be found during a normal operation and consequently involve an excessive tensioning of the yarns resulting in structural defects of the finished product.